1. Technical Field
The present invention relates to a method for manufacturing an inner cutter for a reciprocating electric shaver in which an inner cutter makes reciprocating motions while making sliding contact with the inside surface of an arch-shaped outer cutter and further relates to such an inner cutter.
2. Description of the Related Art
In a typical reciprocating electric shaver, the inner cutter is caused to make a reciprocating motion while making sliding contact with the inside surface of an arch-shaped outer cutter, thus cutting by the inner cutter hair that advances into the apertures formed in the outer cutter. Such inner cutters include an assembled inner cutter and an integral inner cutter as disclosed in Japanese Patent Application Laid-Open (Kokai) No. S62-148684.
In the assembled inner cutter, a plurality of cutter blades formed by stamping a thin metal plate into an arch shape are lined up at fixed intervals and held on a retaining base. In such an inner cutter, it is necessary to form a plurality of cutter blades and attach these cutter blades to a cutter blade attachment member. Accordingly, it requires increased numbers of manufacturing steps, and the problem is its poor productivity.
To the contrary, the integral inner cutter is a cutter in which all of the cutter blades are integrated. FIGS. 7 and 8 are perspective of such conventional integral inner cutters.
In the inner cutter 10 shown in FIG. 7, a plurality of arch-shaped cutter blades 14 are formed parallel to each other at fixed intervals by slits 12 that are opened in a hollow cylindrical body of a metal, ceramic, etc. (hereafter collectively called a “metal” in the present application) so that the slits 12 are substantially perpendicular to the axis of the hollow cylindrical body. The inner cutter 16 of FIG. 8 includes a plurality of arch-shaped cutter blades 20 that are parallel to each other at fixed intervals, and these cutter blades 20 are obtained by bending a flat metal plate into substantially an arch shape and by forming slits 18 that cut across the ridge line of the arch-shaped metal plate.
Of the above-described inner cutters, the inner cutter 10 shown in FIG. 7 is obtained by groove-cutting that is done by, as shown in FIG. 9, moving a circular cutting tool 200, which is rotated as shown by two-head curved arrow, in the direction perpendicular to the axis of a metallic hollow cylindrical body 10A as shown by two-head straight arrow, thus forming the slits 12. On the other hand, in the arch-shaped inner cutter 16 shown in FIG. 8, a thin metal plate formed with cutter blades 20 is used instead of the hollow cylindrical body 10A and bent into an arch shape.
However, in the method that uses a rotating cutting tool 200 as shown in FIG. 9, the cutter blades 14 (20) take the sectional shape as shown in FIG. 10. FIG. 10 is a sectional view of the inner cutter 10 (16) in a perpendicular section that includes the ridge line (and the centerline in the direction of the reciprocating motion of the inner cutter). In the cutter blades 14 (20) in FIG. 10, the rake angle θ which is the angle formed between the top surface (cutter surface) 22 and the end surfaces 24 directly beneath the top surface 22 of each cutter blade 14 (20) is 90°.
The top surfaces 22 of these cutter blades 14 (20) make a reciprocating motion while making sliding contact with the inside surface of an arch-shaped outer cutter 26 and thus cut hair that enters through apertures formed in the outer cutter 26. Accordingly, it is desirable that the rake angle θ be as sharp as possible; in other words, it is desirable that the rake angle θ be an acute angle that is less than 90°.
In order to form the rake angle θ into an acute angle, the outer end surfaces 24 of the cutter blades 14 (20) are ground or polished (hereafter collectively referred to simply as “grinding” in some cases) using a grindstone 28 or 30 as shown in FIG. 11. The grindstone 28 of a disk-form or rod-form grindstone is inserted into the spaces (slits 12 (18)) between the arch-shaped cutter blades 14 (20) and rotated as shown by two-head curved arrow and moved. The grindstone 30, on the other hand, has a tip end of a knob shape, and grinding is performed by rotating as shown by circular arrow and moving this knob-shaped tip end while pressing the knob-shaped tip end against the end surfaces 24.
FIG. 12 shows another way to form an acute rake angle in cutter blades. In the method of FIG. 12, grinding is performed by a circular grindstone 32 which is thicker than the width of the slits 18 of the arch-shaped inner cutter 16 and whose circumferential edge protrudes in the form of an acute angle; and such a circular grindstone 32 is caused to advance into the slits 18 from the inside of the inner cutter 16 while being rotated. This method is disclosed in Japanese Patent Application Laid-Open (Kokai) No. S53-116961. In FIG. 12, the reference numeral 34 is a centerline of the rotation of the grindstone 32.
In the inner cutters 10 and 16 made by the methods illustrated in FIGS. 7 through 9, since a thin metal plate or a metallic hollow cylindrical body with a certain thickness is used, the thickness of the cutter blades 14 and 20 (i.e., the thickness in the radial direction) is the same as the thickness of the thin metal plate or of the hollow cylindrical body. Ordinarily, it is necessary to reduce the weight of the inner cutter in order to reduce the driving force of the driving motor of a shaver and thus to reduce the consumption of energy; furthermore, it is desirable to avoid a thin metal plate or a hollow cylindrical body that has an excessive thickness in order to improve the grindability.
On the other hand, if a thin metal plate or a hollow cylindrical body of a small thickness is used, then the strength of the cutter blades becomes insufficient, and the inner cutter is caused to flex repeatedly together with the outer cutter by the pressure that is applied to the outer cutter during shaving. Further, the cutter blades undergo metal fatigue as a result of deformations, sagging of the cutter tips occurs, and the problem of deterioration in sharpness arises. Though inner cutters in which resin molded parts used for reinforcement are attached to compensate for the insufficient strength exist, the number of parts increases in such inner cutters, and the weight of the reciprocating portion also increases.
The method in which, as shown in FIG. 11, the end surfaces of the arch-shaped cutter blades 14 (20) are ground by causing the grindstone 28 or 30 to advance into the slits 12 (18) from the outside to make the rake angle θ of the cutter blades in acute angles requires extremely fine work, and thus it requires long working time. As a result, the working efficiency is poor, and the problem of productivity drop and manufacturing yield arises.
Furthermore, in the method that uses a rotating grindstone 32 as shown in FIG. 12, the grindstone 32 is caused to advance into the slits 18 from the inside of the arch-shaped inner cutter 16. Accordingly, the grindstone 32 needs to have an extremely small diameter. However, small diameter grindstones tend to easily wear out in a short period of time, and it is necessary to frequently replace the grindstones. As a result, in the method shown in FIG. 11, the working efficiency is poor, and the problem of a high manufacturing cost arises.